1. Field of the Invention
The present invention relates to an OLED panel having an organic OLED (OLED: Organic Light Emitting Diode) formed on a substrate and sealed between the substrate and a cover member, and also to an OLED module mounting an IC or the like including a controller on such an OLED panel. In the present description, both an OLED panel and an OLED module are, collectively, referred to as a light-emitting device. The present invention, furthermore, relates to a driving method to a light-emitting device and an electronic apparatus using such a light-emitting device.
2. Description of the Related Art
The OLED, spontaneous to emit light, provides high visibility but does not require such a backlight as needed on a liquid-crystal display (LCD) thus optimally reducing the thickness, which is furthermore limitless in viewing angle. Consequently, the light-emitting devices using OLEDs have recently drawn attentions as the display devices taking the place of CRTs and LCDs.
The OLED has a layer containing an organic compound (organic light-emitting material) to cause electroluminescence under the application of an electric field (hereinafter, described as organic light-emitting layer), an anode and a cathode. The electroluminescence on an organic compound includes the emission of light of upon returning from a singlet excitation state into the ground state (fluorescence) and the emission of light of upon returning from a triplet excitation state into the ground state (phosphorescence). The light-emitting device of the present invention may use either one or both of such emission of light.
In the description, every layer provided between an OLED cathode and an anode is defined as an organic light-emitting layer. The organic light-emitting layer, concretely, includes a light-emitting layer, a hole injection layer, an electron injection layer, a hole transport layer and an electron transport layer. Basically, the OLED has a structure layered with an anode, a light-emitting layer and a cathode in the order. In addition to this structure, some structures possess an anode, a hole injection layer, a light-emitting layer and a cathode or an anode, a hole injection layer, a light-emitting layer, an electron transport layer and a cathode in the order.
It is problematic in placing the light-emitting device into practical application that the brightness of an OLED lowers due to deterioration in the organic light-emitting material.
The organic light-emitting material is less resistive to moisture, oxygen, light and heat, and to be acceleratingly deteriorated by them. Specifically, the rate of deterioration is dependent upon a device structure for driving the light-emitting device, organic light-emitting material properties, electrode materials, conditions in a fabrication process, a driving scheme to the light-emitting device and so on.
With even a constant voltage to the organic light-emitting layer, if the organic light-emitting layer deteriorates, OLED brightness lowers. This results in obscured image display. Note that, in the description, the voltage applied from a pair of electrodes to an organic light-emitting layer is defined as an OLED drive voltage (Vel).
Meanwhile, in a color display scheme using three kinds of OLEDs corresponding to R (red), G (green) and B (blue), the organic light-emitting materials forming the organic light-emitting layer are different between the colors to which the OLEDs correspond. Consequently, there is a possibility that the OLED organic light-emitting layer deteriorates at a different rate dependently upon the color. In this case, as time elapses the brightness of OLED becomes different by the color, making impossible for the light-emitting device to display an image with a desired color.
Meanwhile, the temperature of the organic light-emitting layer relies upon the temperature of outside air or the heat generated by the OLED panel itself. However, the OLED generally has a flowing current value varying with temperature. FIG. 27 shows a change of a voltage-current characteristic of an OLED when changing the temperature of the organic light-emitting layer. At a constant voltage, when the temperature of the organic light-emitting layer increases, the OLED drive current increases. Because the OLED drive current and the OLED brightness are in a proportional relationship, the brightness on the OLED increases with the increase in the OLED drive current. In this manner, because OLED brightness varies with the temperature of the organic light-emitting layer, display is difficult at a desired gray scale. The consumption current of the light-emitting device increases with the rise of temperature.
Furthermore, because generally the change rate of OLED drive current against temperature change is different depending on the kind of an organic light-emitting material, there is a possibility in color display that the OLED brightness of each color dependently varies with temperature. The brightness balance, if disordered between the colors, makes impossible to display in a desired color.
It is an object of the present invention to provide a light-emitting device capable of obtaining a constant brightness regardless of deterioration in an organic light-emitting layer or temperature change and further of providing display with a desired color.
The present inventor has paid attentions to the fact that the lowering in OLED brightness due to deterioration is less when light is emitted while keeping the current flowing through the OLED at constant rather than when light is emitted while keeping the OLED drive voltage at constant. Note that, in the description, the current flowing through an OLED is referred to as an OLED drive current (Iel). It has been considered that the OLED brightness change due to OLED deterioration can be prevented due to control of the OLED brightness not by a voltage but by a current.
Specifically, the present invention controls the drain current Id of a transistor provided in each pixel by a signal line drive circuit. Because the transistor drain current Id is controlled in the signal line drive circuit, the drain current Id is placed constant regardless of the value of a load resistance.
When a drain current Id flows, a voltage occurs between the gate electrode and the drain region of the transistor. While maintaining the voltage, the transistor drain current is caused to flow to the OLED through a singular or a plurality of circuit elements. Incidentally, the drain current Id is in such a magnitude as operating the transistor in a saturation region.
By the above configuration, the OLED drive current flowing through the OLED is controlled in value by the signal line drive circuit regardless of the value of a load resistance. In other words, the OLED drive current can be controlled to a desired value without being affected by transistor characteristic difference, OLED deterioration or the like.
The present invention can suppress the lowering in OLED brightness by the above configuration even where an organic light-emitting layer deteriorates. As a result, a clear image can be displayed. Meanwhile, in the case of a color-display light-emitting device using OLEDs corresponding to respective colors, if the OLED organic light-emitting layer deteriorates at a rate different between the colors prevents the colors from being unbalanced in brightness, thereby displaying a desired color.
Meanwhile, even where the temperature of the organic light-emitting layer is influenced by outside-air temperature or the heat generated by the OLED panel itself, the OLED current can be controlled to a desired value. Accordingly, because the OLED drive current and the OLED brightness are in proportion, the OLED brightness can be suppressed from changing. Furthermore, consumption current can be prevented from increasing due to temperature rise. In the case of a color-display light-emitting device, the colors are suppressed from changing in OLED brightness without being affected by temperature change. Accordingly, the colors can be prevented from being unbalanced in brightness thereby displaying a desired color.
Furthermore, because generally the change rate of OLED drive current due to temperature change is different depending on the kind of an organic light-emitting material, there is a possibility that the colors in color display variously change in OLED brightness depending on temperature. However, the light-emitting device of the present invention can obtain a desired brightness without being affected by temperature change. Accordingly, the colors are prevented from being unbalanced in brightness thus displaying a desired color.
Meanwhile, the general light-emitting device has an electric resistance on the wiring to supply current to each pixel, and hence the potential thereon somewhat drops due to wiring length. The potential drop largely differs depending on an image to be displayed. Particularly, in a plurality of pixels to be supplied by a current from the same wiring, where the ratio of the pixels having many gray scales increases, the current flowing the wiring increases to conspicuously cause potential drop. The potential drop decreases the voltage to be applied to the OLED of each pixel, hence decreasing the current to be supplied to the pixel. Accordingly, in the case to provide display in a constant gray scale on a certain predetermined pixel, when there is change in gray scale on another pixel being supplied with current through the same wiring, this causes a change in the current to be supplied to the predetermined pixel, resultingly changing the gray scale. However, the light-emitting device of the present invention can obtain a measurement value and reference value on each display image thereby correcting the OLED current. Accordingly, even where there is change in a display image, it can be displayed at a desired gray scale by the correction.
Incidentally, in the light-emitting device of the present invention, the transistor for use in the pixel may be a transistor formed using single-crystal silicon or a thin-film transistor using polysilicon or amorphous silicon. Otherwise, the transistor may use an organic semiconductor.
Incidentally, the transistor provided on the pixel of the light-emitting device of the present invention may be of a single-gate structure or a multi-gate structure, such as a double-gate structure or the higher.